Boilers are routinely provided for producing hot water for washing and bathing, and also for supply to space heating systems, such as radiators or under floor heating. Within a boiler a fuel is combusted to generate hot gas and this gas passes over or around a heat exchanger in order to warm a liquid, often water, within the heat exchanger. A boiler is designed for an expected heat output. It can intuitively be seen that a large boiler has a large “thermal mass” so using a large boiler having a large heat output would be inefficient if only a small heating load is to be serviced. Similarly a small boiler having a relatively small heat output wont be able to adequately supply the heating needs of a large building.